Jeremy Corbyn has started work on putting together his shadow cabinet after his dramatic landslide victory in the Labour leadership contest.
The veteran left winger - who has never held a formal position in the party before now - must also prepare for his first Commons clash with David Cameron.
The new Labour leader has promised to "unite" the party after getting 60% of the votes in the leadership contest.
Mr Corbyn's victory has sparked an exodus of shadow cabinet members.
But senior figures, including his predecessor Ed Miliband and former deputy prime minister Lord Prescott urged MPs to get behind Mr Corbyn because he had a strong mandate from party members.
The Islington North MP is facing calls to reach out to all sections of the party - not just the small group of left-wing MPs who supported his candidacy.
Mr Corbyn is expected to name the first members of his new shadow cabinet later.
Defeated rivals Yvette Cooper and Liz Kendall, shadow education secretary Tristram Hunt, shadow work and pensions secretary Rachel Reeves and shadow chancellor Chris Leslie, are among those who have said they will not serve under him.
But defeated leadership contender and shadow health secretary Andy Burnham has refused to comment and shadow business secretary Chuka Umunna, who pulled out of the contest at an early stage, said the party had to "come together" behind its new leader.
Mr Corbyn began the three-month contest as a rank outsider but was swept to victory on a wave of enthusiasm for his anti-austerity message and promise to scrap Britain's nuclear weapons and renationalise the railways and major utilities.
In a speech to supporters at a London pub after his victory, Mr Corbyn said there would be "huge battles ahead," adding "we need to democratise the way decisions are made in the party".
Mr Corbyn, who has spent his entire 32 year career on the backbenches, said he wanted to build a society based on "socialist values" but acknowledged his job as Labour leader was going to be "a complicated one".
He is likely to continue to appear at public demonstrations, changing the focus of the Labour Party with his promise to reach out to younger supporters.
Analysis by BBC Political Editor Laura Kuenssberg
There are problems everywhere for Labour's new leader. He has always been an outsider, an insurgent in his own party.
How can he expect loyalty from his colleagues, unite the party, when he has rarely displayed it himself?
MPs have been discussing ousting him for weeks. There will likely be initial faint support from most. Don't expect a rapid coup.
But don't doubt most smiles behind him at the despatch box will be through gritted teeth.
And shadow ministers' resignation letters have already been written.
He has also hinted that he wants to change the format of Prime Minister's Questions - he faces David Cameron across the despatch box for the first time on Wednesday - suggesting other Labour MPs might get a turn.
His first campaign will be spent opposing government plans to "shackle" trade unions by imposing higher thresholds for strike ballots, Mr Corbyn said.
The bill - which proposes a 50% threshold in union strike ballots, a clampdown on picketing, allowing firms to employ agency staff to cover for strikers - will have its second reading in the Commons on Monday.
Mr Corbyn spoke out against the proposals in his acceptance speech, saying Labour was organically linked to the union movement.
He also plans to address the TUC conference in Brighton on Tuesday, where he is likely to receive a hero's welcome from trade unionists, who voted for him in overwhelming numbers.
Veteran Labour MP Frank Field - one of those who nominated Mr Corbyn - told the BBC the party was facing a crisis.
"I don't think Jeremy, despite this extraordinary win, will lead us anywhere other than into a cul-de-sac, but I do not believe those who are uneasy about Jeremy's leadership have an alternative and a coherent set of policies which would win us an election," he said.
Mr Field said he helped Mr Corbyn get onto the leadership ballot in the hope of creating a debate between the left and what he called "mainstream Labour", but he said that had failed dismally.
Former Labour cabinet minister Lord Mandelson - who has previously spoken out against Mr Corbyn - said Labour Party members were facing the "test of our political lives".
Writing in the Sunday Times, he said Labour could now become a "party on the fringes", and would "slide into history" unless it had a leader with policies that voters regarded as "relevant and workable".
Mr Corbyn was announced as the party's new leader on Saturday.
He won on the first round of voting in the leadership contest, taking 251,417 of the 422,664 votes cast - against 19% for Mr Burnham, 17% for Ms Cooper and 4.5% for Liz Kendall. Former minister and Gordon Brown ally Tom Watson was elected deputy leader.
Mr Corbyn also topped the ballot among Labour members and the thousands who signed up to vote for £3 - 85% of whom were Corbyn supporters.
Defence Secretary Michael Fallon, giving the Conservative Party's reaction, said: "Labour are now a serious risk to our nation's security, our economy's security and your family's security."